Hi,
I don't think this error has anything to do with the KeepAlive in mojoPortal. I think something or someone else is trying to access that page on your site and it doesn't exist so the mojo writes an error about it and states that it couldn't find virtual page for it either.
Some other CMS' I have used have a KeepAlive.aspx file, have you used a different CMS on this domain in the past? If so, maybe you setup some kind of service to use the KeepAlive page. I know DNN has (or had) a feature like this at one time.
HTH,
Joe D.